(1) Field of the Invention
The present invention relates to an image output apparatus for gradation image data processed in an image processing apparatus, particularly, an image output apparatus in which the gradation level of each pixel can be controlled.
(2) Description of the Related Art
Image data processed in a data processing apparatus is typically output to image output apparatuses such as a graphic display apparatus or a graphic printing apparatus.
Image output apparatuses are either a multi-bit gradation image output apparatus type or a binary image output apparatus type. In the multi-bit gradation image output apparatus, each pixel is defined by a plurality of data bits and can have a plurality of gradation levels corresponding to the data. In the binary image output apparatus, each pixel is defined by one data bit and can only have an ON or OFF state. Therefore, a gradation image can be output in the multi-bit gradation image output apparatus, however, a gradation image cannot be directly output in the binary image output apparatus.
In the multi-bit gradation image output apparatus, the gradation level of each pixel is required to be changed according to input data of the pixel. For example, when a cathode ray tube (CRT) is used as a multi-bit gradation image output display, the intensity of an electron beam can be changed at every pixel. When a laser printer is used as a multi-bit gradation image output printing apparatus, the intensity of a laser beam can be changed at every pixel and the density of each pixel is changed in proportion with the intensity of the laser beam.
In the binary image output apparatus, each pixel is only able to have two sates, ON and OFF. For example, in a printing apparatus, each pixel has one of two states, namely, whether ink is applied to an area of the pixel.
When the image output apparatus receives image data sent from an image processing apparatus such as a host computer, the image data is expanded in an image memory of the image output apparatus. Thereafter, the image data is output to a hardware device which converts image data into an actual image. The image memory has a matrix of memory bits corresponding to a whole image plane or parts of a whole image plane. The image memories are so-called bit-maps. Each pixel of binary image data is defined by one bit of data, however, each pixel of multi-bit image data is represented by a plurality of bits data, necessary to represent a number of gradation levels. Therefore, the memory capacity of the bit-map of the multi-bit image output apparatus is much larger than that of a binary image output apparatus of same bit-map size. Thus, the processing quantity of image bits of same bit-map size becomes larger in the multi-bit image output apparatus than in the binary image output apparatus. Consequently, the multi-bit image output apparatus is generally expensive compared with the binary image output apparatus.
There has been developed a method for representing gradation image in the binary bit image output apparatus. In this method, every apparent pixel unit is composed of a plurality of primitive (actual) pixels of the output apparatus and the gradation level of each unit is determined according to a ratio of ON primitive pixels included in the unit. In this specification, this method is called "1-bit gradation image method". The well-known "dither" method is a modified 1-bit gradation image method. In the 1-bit gradation image method, although a number of representable gradation levels increases in proportion to an increase of a number of primitive pixels included in a unit, resolution of the gradation image decreases.
For example, when an apparent pixel is composed of sixteen primitive pixels included in 4.times.4 square, 4.sup.2 +1, namely seventeen gradation levels can be represented, however, the line resolution is reduced to 1/4.
As described above, since the multi-bit gradation image output apparatus is expensive, the binary image output apparatus is widely used. Therefore, the 1-bit gradation method is widely used to represent a gradation image with the binary image output apparatus.
In a data processing system, processed results are printed onto cutforms and documents on which particular characters and frame lines are previously printed. These previously printed characters and frame lines are generally printed at low density in order to easily discriminate them from characters and numerals printer later.
Thus, in order to use normal blank papers, the previously printed characters and frame lines are required to be simultaneously printed with characters and numericals of the processed results.
When this printing operation is carried out in the binary image output apparatus, characters and frame lines are printed by the 1-bit gradation method. As described above, the representing resolution deceases when the gradation levels are represented by the 1-bit gradation method, therefore, small character and a narrow line of low density cannot be finely represented.
This problem can be avoided by using a multi-bit image output apparatus. However, as described above, since the bit-map memory of the multi-bit image output apparatus is large, the processing quantity becomes large. This causes a problem that the processing speed decreases when images other than multi-bit gradation images are printed.
Further, there are already plenty of images made by the 1-bit gradation method. When these images are reproduced by the 1-bit gradation method and output to the multi-bit image output apparatus, the multi-bit gradation image output apparatus is used as a binary gradation image output apparatus in which only two, the maximum and minimum gradation levels, of many gradation levels are used. Therefore, the resolution does not increase, and the processing quantity becomes large. For example, when image data represented by the 1-bit gradation method is output in a multi-bit gradation image output apparatus, in which each pixel can be 256 gradation levels represented by 8 bits data, 00 or FF hexadecimal bits are transferred to the memory area corresponding to each pixel as gradation data. If this transfer operation is carried out by a 16-bit data processor, data for two pixels is simultaneously transferred. When binary image data is transferred by a 16-bit data processor, data for 16 pixels are simultaneously transferred since each pixel has 1-bit data. Namely, when the multi-bit image data output apparatus is used, 8-times more processing capacity is required.
In this way, the multi-bit image data output apparatus becomes necessary when a gradation image cannot be represented in the binary image data output apparatus. However, when binary image data is output, a problem that processing speed decreases occurs.
Further, since the multi-bit image data output apparatus includes a large memory capacity, it is desired to reduce memory capacity of the multi-bit image data output apparatus.